U.S. patent application number 15/300345 was filed with the patent office on 2017-06-29 for contention window adaptation in multi-carrier listen-before-talk protocols.
The applicant listed for this patent is Telefonaktiebolaget LM Ericsson (Publ). Invention is credited to Jung-Fu CHENG, Sorour FALAHATI, Laetitia FALCONETTI, Du Ho KANG, Reem KARAKI, Havish KOORAPATY, Daniel LARSSON, Amitav MUKHERJEE.
Application Number | 20170188387 15/300345 |
Document ID | / |
Family ID | 56561411 |
Filed Date | 2017-06-29 |
United States Patent
Application |
20170188387 |
Kind Code |
A1 |
MUKHERJEE; Amitav ; et
al. |
June 29, 2017 |
CONTENTION WINDOW ADAPTATION IN MULTI-CARRIER LISTEN-BEFORE-TALK
PROTOCOLS
Abstract
A method and network node for adaptation of contention windows
in a multicarrier wireless communication system implementing a
listen-before-talk protocol are disclosed. According to one aspect,
a method includes determining at least one component carrier (CC),
of multiple CCs to serve as a backoff channel. The method further
includes performing a listen-before-talk procedure on the at least
one CC serving as a backoff channel. The listen-before-talk
procedure includes sensing for each backoff channel whether a clear
channel exists during a backoff period drawn from a contention
window (CW). The LBT procedure also includes deferring transmitting
on a CC for which the sensing does not indicate that a clear
channel exists. The LBT procedure also includes transmitting on a
CC for which the sensing indicates a clear channel exists. The
method also includes determining a size of the CW based on at least
one transmission feedback value.
Inventors: |
MUKHERJEE; Amitav; (Fremont,
CA) ; CHENG; Jung-Fu; (Fremont, CA) ;
FALAHATI; Sorour; (Stockholm, SE) ; FALCONETTI;
Laetitia; (Solna, SE) ; KANG; Du Ho;
(Sollentuna, SE) ; KARAKI; Reem; (Aachen, DE)
; KOORAPATY; Havish; (Saratoga, CA) ; LARSSON;
Daniel; (Stockholm, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Telefonaktiebolaget LM Ericsson (Publ) |
Stockholm |
|
SE |
|
|
Family ID: |
56561411 |
Appl. No.: |
15/300345 |
Filed: |
July 8, 2016 |
PCT Filed: |
July 8, 2016 |
PCT NO: |
PCT/SE2016/050708 |
371 Date: |
September 29, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62204868 |
Aug 13, 2015 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L 5/0055 20130101;
H04W 74/002 20130101; H04W 74/0808 20130101; H04L 1/1812
20130101 |
International
Class: |
H04W 74/08 20060101
H04W074/08; H04L 5/00 20060101 H04L005/00; H04L 1/18 20060101
H04L001/18 |
Claims
1. A method of adaptation of contention windows in a multicarrier
wireless communication system implementing a listen-before-talk
protocol, the method comprising: determining at least one component
carrier, CC, of multiple CCs to serve as a backoff channel;
performing a listen-before-talk procedure on the at least one CC
serving as a backoff channel, the listen-before-talk procedure
including: sensing for each backoff channel whether a clear channel
exists during a backoff period drawn from a contention window, CW;
deferring transmitting on a CC for which the sensing does not
indicate that a clear channel exists; and transmitting on a CC for
which the sensing indicates a clear channel exists; and determining
a size of the CW based on at least one transmission feedback
value.
2. The method of claim 1, wherein the listen-before-talk procedure
further includes: performing a clear channel assessment, CCA, on
CCs that do not serve as a backoff channel; and transmitting on CCs
for which a CCA indicates a clear channel.
3. The method of claim 1, wherein only one CC serves as a backoff
channel.
4. The method of claim 1, wherein the transmission feedback value
is a Hybrid Automatic Repeat Request, HARQ, transmission feedback
value and the CW is increased only if a ratio of negative
acknowledgments, NACKs, to acknowledgements, ACKs, on each
component carrier, CC, exceeds a threshold.
5. The method of claim 1, wherein the transmission feedback value
is a Hybrid Automatic Repeat Request, HARQ, transmission feedback
value and the CW is increased only if a ratio of negative
acknowledgments, NACKs, to acknowledgements, ACKs, on all backoff
channels exceeds a threshold.
6. The method of claim 1, wherein an increase of the size of the CW
is obtained by multiplication of a CW by a factor greater than
one.
7. The method of claim 1, wherein the backoff period is drawn from
a joint contention window, JCW, that is determined from CWs of the
multiple component carriers.
8. The method of claim 7, wherein the JCW is a maximum of the CWs
of the multiple component carriers.
9. The method of claim 7, wherein the JCW is an average of the CWs
of the multiple component carriers.
10. The method of claim 1, wherein the CW is increased if at least
one transmission on a CC results in a negative-acknowledgment,
NACK, signal.
11. A network node for adaptation of contention windows in a
multicarrier wireless communication system implementing a
listen-before-talk protocol, the network node comprising:
processing circuitry including: a processor; a memory in
communication with the processor, the memory including executable
instructions that, when executed by the processor, configure the
processor to: determine at least one component carrier, CC, of
multiple CCs to serve as a backoff channel; perform a
listen-before-talk procedure on the at least one CC serving as a
backoff channel, the listen-before-talk procedure including:
sensing for each backoff channel whether a clear channel exists
during a backoff period drawn from a contention window, CW
deferring transmitting on a CC for which the sensing does not
indicate that a clear channel exists; and determine a size of CW
based on at least one transmission feedback value; and a
transmitter configured to transmit on CCs for which the sensing
indicates a clear channel exists.
12. The network node of claim 11, wherein the listen-before-talk
procedure further includes: performing a clear channel assessment,
CCA, on CCs that do not serve as a backoff channel; and
transmitting on CCs for which a CCA indicates a clear channel.
13. The network node of claim 11, wherein only one CC serves as a
backoff channel.
14. The network node of claim 11, wherein the transmission feedback
value is a Hybrid Automatic Repeat Request, HARQ, transmission
feedback value and CW is increased only if a ratio of negative
acknowledgments, NACKs, to acknowledgements, ACKs, on each backoff
channel exceeds a threshold.
15. The network node of claim 11, wherein the transmission feedback
value is a Hybrid Automatic Repeat Request, HARQ, transmission
feedback value and the CW is increased only if a ratio of negative
acknowledgments, NACKs, to acknowledgements, ACKs, on all backoff
channel exceeds a threshold.
16. The network node of claim 11, wherein an increase of the size
of the CW is obtained by multiplication of a CW by a factor greater
than one.
17. The network node of claim 11, wherein the backoff period is
drawn from a joint contention window, JCW, that is determined from
CWs of the multiple component carriers.
18. The network node of claim 17, wherein the JCW is a maximum of
the CWs of the multiple component carriers.
19. The network node of claim 17, wherein the JCW is an average of
the CWs of the multiple component carriers.
20. The network node of claim 11, wherein the CW is increased if at
least one transmission on a CC results in a
negative-acknowledgment, NACK, signal.
21. A network node for adaptation of contention windows in a
multicarrier wireless communication system implementing a
listen-before-talk protocol, the network node comprising: a
listen-before-talk module, configured to perform a
listen-before-talk, LBT, procedure for each of at least one
component carrier, CC, serving as a backoff channel, to determine
whether a clear channel exists on the backoff channel during a
backoff period drawn from a contention window, CW; and a contention
window size determination module configured to determine a size of
the CW based on at least one transmission feedback value; a
transmitter module configured to: transmit on CCs for which the
sensing indicates a clear channel exists; and defer transmitting on
CCs for which the sensing does not indicate that a clear channel
exists.
Description
TECHNICAL FIELD
[0001] The disclosure relates to wireless communication, and in
particular to adaptation of contention windows for multi-carrier
listen-before-talk (LBT) protocols.
BACKGROUND
[0002] The ongoing third generation partnership project (3GPP)
Rel-13 study item, "Licensed-Assisted Access" (LAA), intends to
allow long term evolution (LTE) equipment to also operate in the
unlicensed 5 giga-Hertz (GHz) radio spectrum. The unlicensed 5 GHz
spectrum is used as a complement to the licensed spectrum.
Accordingly, devices connect in the licensed spectrum (primary cell
or PCell) and use carrier aggregation to benefit from additional
transmission capacity in the unlicensed spectrum (secondary cell or
SCell). To reduce the changes required for aggregating licensed and
unlicensed spectrum, the LTE frame timing in the primary cell is
simultaneously used in the secondary cell.
[0003] Regulatory requirements, however, may not permit
transmissions in the unlicensed spectrum without prior channel
sensing. Since the unlicensed spectrum must be shared with other
radios of similar or dissimilar wireless technologies, a so called
listen-before-talk (LBT) method needs to be applied. LBT involves
sensing the medium for a pre-defined minimum amount of time and
backing off if the channel is busy. Today the unlicensed 5 GHz
spectrum is mainly used by equipment implementing the Institute of
Electrical and Electronic Engineers (IEEE) 802.11 Wireless Local
Area Network (WLAN) standard. This standard is also known under its
marketing brand, "Wi-Fi."
[0004] The choice of parameters used in the LBT procedure prior to
accessing the channel has a major impact on inter-radio access
technology (RAT) coexistence and throughput. Of particular
relevance is the method of adapting the size of contention windows
in different random access methods, which determines how long nodes
have to wait before attempting to transmit on the medium.
[0005] LTE uses orthogonal frequency division multiplex (OFDM) in
the downlink and discrete Fourier transform (DFT)-spread OFDM (also
referred to as single-carrier frequency division multiple access
(FDMA)) in the uplink. The basic LTE downlink physical resource can
thus be seen as a time-frequency grid as illustrated in FIG. 1,
where each resource element corresponds to one OFDM subcarrier
during one OFDM symbol interval. The uplink subframe has the same
subcarrier spacing as the downlink and the same number of sub
carrier (SC)-FDMA symbols in the time domain as OFDM symbols in the
downlink.
[0006] In the time domain, LTE downlink transmissions are organized
into radio frames of 10 ms, each radio frame consisting of ten
equally-sized subframes of length Tsubframe=1 ms as shown in FIG.
2. Each subframe comprises two slots of duration 0.5 ms each, and
the slot numbering within a frame ranges from 0 to 19. For normal
cyclic prefix, one subframe consists of 14 OFDM symbols. The
duration of each symbol is approximately 71.4 .mu.s.
[0007] Furthermore, the resource allocation in LTE is typically
described in terms of resource blocks (RB), where a resource block
corresponds to one slot (0.5 ms) in the time domain and 12
contiguous subcarriers in the frequency domain. A pair of two
adjacent resource blocks in time direction (1.0 ms) is known as a
resource block pair. Resource blocks are numbered in the frequency
domain, starting with 0 from one end of the system bandwidth.
[0008] Downlink transmissions are dynamically scheduled, i.e., in
each subframe the base station transmits control information about
which wireless device data is transmitted to and upon which
resource blocks the data is transmitted, in the current downlink
subframe. This control signaling is typically transmitted in the
first 1, 2, 3 or 4 OFDM symbols in each subframe and the number
n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The
downlink subframe also contains common reference symbols, which are
known to the receiver and used for coherent demodulation of, e.g.,
the control information. A downlink system with CFI=3 OFDM symbols
as control is illustrated in FIG. 3.
[0009] From LTE Rel-11 onwards, the above described resource
assignments can also be scheduled on the enhanced Physical Downlink
Control Channel (EPDCCH). For Rel-8 to Rel-10 only the Physical
Downlink Control Channel (PDCCH) is available.
[0010] The reference symbols shown in FIG. 3 are the cell specific
reference symbols (CRS) and are used to support multiple functions
including fine time and frequency synchronization and channel
estimation for certain transmission modes.
[0011] In the LTE system, a wireless device, such as a user
equipment (UE), is notified by the network of downlink data
transmission by the physical downlink control channel (PDCCH). Upon
reception of a PDCCH in a particular subframe, n, a wireless device
is required to decode the corresponding physical downlink shared
channel (PDSCH) and to send acknowledgement/negative acknowledgment
(ACK/NAK) feedback in a subsequent subframe n+k. The ACK/NAK
feedback informs the base station, such as an eNodeB, whether the
corresponding PDSCH was decoded correctly. When the eNodeB detects
an ACK feedback, it can proceed to send new data blocks to the
wireless device. When a NAK is detected by the eNodeB, coded bits
corresponding to the original data block will be retransmitted.
When the retransmission is based on repetition of previously sent
coded bits, it is said to be operating in a Chase combining hybrid
automated repeat request (HARQ) protocol. When the retransmission
contains coded bits unused in previous transmission attempts, it is
said to be operating in an incremental redundancy HARQ
protocol.
[0012] In LTE, the ACK/NAK feedback is sent by the wireless device
using one of the two possible approaches depending on whether the
wireless device is simultaneously transmitting a physical uplink
shared channel (PUSCH):
[0013] If the wireless device is not transmitting a PUSCH at the
same time, the ACK/NAK feedback is sent via a physical uplink
control channel (PUCCH).
[0014] If the wireless device is transmitting a PUSCH
simultaneously, the ACK/NAK feedback is sent via the PUSCH.
[0015] The LTE Rel-10 standard supports bandwidths larger than 20
mega-Hertz (MHz). One important requirement of LTE Rel-10 is to
assure backward compatibility with LTE Rel-8. This should also
include spectrum compatibility. This implies that an LTE Rel-10
carrier wider than 20 MHz should appear as a number of LTE carriers
to an LTE Rel-8 terminal. Each such carrier can be referred to as a
Component Carrier (CC). In particular, for early LTE Rel-10
deployments it can be expected that there will be a smaller number
of LTE Rel-10-capable terminals compared to many LTE legacy
terminals. One should assure an efficient use of a wide carrier
also for legacy terminals, i.e., that it is possible to implement
carriers where legacy terminals can be scheduled in all parts of
the wideband LTE Rel-10 carrier. A straightforward way to obtain
this is by means of Carrier Aggregation (CA). CA implies that an
LTE Rel-10 terminal can receive multiple CC, where the CC have, or
at least the possibility to have, the same structure as a Rel-8
carrier. CA is illustrated in FIG. 4. A CA-capable wireless device
is assigned a primary cell (PCell) which is always activated, and
one or more secondary cells (SCells) which may be activated or
deactivated dynamically.
[0016] The number of aggregated CC as well as the bandwidth of the
individual CC may be different for uplink and downlink. Symmetric
configuration refers to the case where the number of CCs in
downlink and uplink is the same whereas an asymmetric configuration
refers to the case that the number of CCs is different. It is
noteworthy that the number of CCs configured in a cell may be
different from the number of CCs seen by a terminal: A terminal may
for example, support more downlink CCs than uplink CCs, even though
the cell is configured with the same number of uplink and downlink
CCs. Each component carrier operates its own individual HARQ
instance.
[0017] In typical deployments of WLAN, carrier sense multiple
access with collision avoidance (CSMA/CA) is used for medium
access. This means that the channel is sensed to perform a clear
channel assessment (CCA), and a transmission is initiated only if
the channel is declared as Idle. In case the channel is declared as
Busy, the transmission is essentially deferred until the channel is
deemed to be Idle.
[0018] A general illustration of the listen before talk (LBT)
mechanism of Wi-Fi is shown in FIG. 5. After a Wi-Fi station A
transmits a data frame to a station B, station B shall transmit the
ACK frame back to station A with a delay of 16 .mu.s corresponding
to a short interframe space (SIFS). Such an ACK frame is
transmitted by station B without performing a LBT operation. To
prevent another station interfering with such an ACK frame
transmission, a station shall defer for a duration of 34 .mu.s
(referred to as a distributed coordination function interframe
space (DIFS)) after the channel is observed to be occupied before
assessing again whether the channel is occupied. Therefore, a
station that wishes to transmit first performs a clear channel
assessment (CCA) by sensing the medium for a fixed duration DIFS.
If the medium is idle then the station assumes that it may take
ownership of the medium and begin a frame exchange sequence. If the
medium is busy, the station waits for the medium to go idle, defers
for DIFS, and waits for a further random backoff period.
[0019] In the above basic protocol, when the medium becomes
available, multiple Wi-Fi stations may be ready to transmit, which
can result in collision. To reduce collisions, stations intending
to transmit select a random backoff counter and wait, i.e.,
backoff, for that number of slot channel idle times. This waiting
is the backoff period. A component carrier upon which a CCA is made
and for which the delay of transmission is applied is referred to
herein as a backoff channel. The amount of the delay is random
according to a random backoff counter. The random backoff counter
is selected as a random integer drawn from a uniform distribution
over the interval of [0, C] where C is a length in integers of a
contention window (CW). The random backoff counter establishes the
backoff period. The default size of the contention window, CWmin,
is set in the IEEE specifications referred to above. Note that
collisions can still happen even under this random backoff protocol
when there are many stations contending for the channel access.
Hence, to avoid recurring collisions, the size of the contention
window is doubled whenever the station detects a collision of its
transmission up to a limit, CWmax, also set in the IEEE
specifications. When a station succeeds in a transmission without
collision, it resets its contention window size back to the default
value CWmin.
[0020] For multi-carrier operation, Wi-Fi follows a hierarchical
channel bonding scheme to determine its transmission bandwidth for
a frame, which could be 20 MHz, 40 MHz, 80 MHz, or 160 MHz, for
example. In the 5 GHz band, wider Wi-Fi channel widths of 40 MHz,
80 MHz, 160 MHz or 80+80 MHz are formed by combining contiguous 20
MHz sub-channels in a non-overlapping manner A pre-determined
primary channel performs the CW-based random access procedure after
a deferral period if necessary, and then counts down the random
number generated. This deferral period is therefore not the same as
the backoff period. The secondary channels only perform a quick
clear channel assessment (CCA) check for a point coordination
function interframe space (PIFS) duration (generally 25 ps) before
the potential start of transmission to determine if the additional
secondary channels are available for transmission. Based on the
results of the secondary CCA check, transmission is performed on
the larger bandwidths; otherwise transmission falls back to smaller
bandwidths. The Wi-Fi primary channel is always included in all
transmissions, i.e., transmission on secondary channels alone is
not allowed.
[0021] For a device not utilizing the Wi-Fi protocol, EN 301.893,
v. 1.7.1 provides the following requirements and minimum behavior
for the load-based clear channel assessment. [0022] 1) Before a
transmission or a burst of transmissions on an Operating Channel,
the equipment shall perform a Clear Channel Assessment (CCA) check
using "energy detect". The equipment shall observe the Operating
Channel(s) for the duration of the CCA observation time which shall
be not less than 20 .mu.s. The CCA observation time used by the
equipment shall be declared by the manufacturer. The Operating
Channel shall be considered occupied if the energy level in the
channel exceeds the threshold corresponding to the power level
given in point 5 below. If the equipment finds the channel to be
clear, it may transmit immediately (see point 3 below). [0023] 2)
If the equipment finds an Operating Channel occupied, it shall not
transmit in that channel The equipment shall perform an Extended
CCA check in which the Operating Channel is observed for the
duration of a random factor N multiplied by the CCA observation
time. N defines the number of clear idle slots resulting in a total
idle period that needs to be observed before initiation of the
transmission. This period is referred to as a backoff period and is
typically random. Thus, the value of N shall be randomly selected
in the range 1 . . . q every time an Extended CCA is required and
the value stored in a random backoff counter. The value of q is
selected by the manufacturer in the range 4 . . . 32. This selected
value shall be declared by the manufacturer (see clause 5.3.1 q of
European Telecommunication Standards Institute (ETSI) EN 301
893V1.7.1 (2012-06)). The random backoff counter is decremented
every time a CCA slot is considered to be "unoccupied". When the
random backoff counter reaches zero, the equipment may transmit.
[0024] NOTE 1: The equipment is allowed to continue Short Control
Signaling Transmissions on this channel providing it complies with
the requirements in clause 4.9.2.3 of ETSI EN 301 893V1.7.1
(2012-06). [0025] NOTE 2: For equipment having simultaneous
transmissions on multiple (adjacent or non-adjacent) operating
channels, the equipment is allowed to continue transmissions on
other Operating Channels providing the CCA check did not detect any
signals on those channels. [0026] 3) The total time that an
equipment makes use of an Operating Channel is the Maximum Channel
Occupancy Time which shall be less than (13/32).times.q ms, with q
as defined in point 2 above, after which the device shall perform
the Extended CCA described in point 2 above. [0027] 4) The
equipment, upon correct reception of a packet which was intended
for this equipment, can skip CCA and immediately (see Note 3,
below) proceed with the transmission of management and control
frames (e.g. ACK and Block ACK frames). A consecutive sequence of
transmissions by the equipment, without it performing a new CCA,
shall not exceed the Maximum Channel Occupancy Time as defined in
point 3 above. [0028] NOTE 3: For the purpose of multi-cast, the
ACK transmissions (associated with the same data packet) of the
individual devices are allowed to take place in a sequence [0029]
5) The energy detection threshold for the CCA shall be proportional
to the maximum transmit power (PH) of the transmitter: for a 23 dBm
equivalent isotropically radiated power (e.i.r.p.) transmitter the
CCA threshold level (TL) shall be equal or lower than-73 decibel
power ratio (dBm)/MHz at the input to the receiver (assuming a 0 dB
isotropic (dBi) receive antenna). For other transmit power levels,
the CCA threshold level TL shall be calculated using the formula:
TL=-73 dBm/MHz+23 -PH (assuming a 0 dBi receive antenna and PH
specified in dBm e.i.r.p.). An example to illustrate the EN 301.893
LBT is provided in FIG. 6, where X represents a failed CCA, and
where, in one embodiment, each CCA of the sequence of CCA checks
occupies a 9 micro-second slot.
[0030] Up to now, the spectrum used by LTE is dedicated to LTE.
This has the advantage that the LTE system does not need to care
about the coexistence issue and the spectrum efficiency can be
maximized However, the spectrum allocated to LTE is limited which
cannot meet the ever increasing demand for larger throughput from
applications/services. Therefore, a new study item has been
initiated in 3GPP on extending LTE to exploit unlicensed spectrum
in addition to licensed spectrum. Unlicensed spectrum can, by
definition, be simultaneously used by multiple different
technologies. Therefore, LTE should consider coexistence with other
systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same
manner in the unlicensed spectrum as in the licensed spectrum can
seriously degrade the performance of Wi-Fi, as Wi-Fi will not
transmit once it detects the channel is occupied.
[0031] One way to utilize the unlicensed spectrum reliably is to
transmit essential control signals and channels on a licensed
carrier. That is, as shown in FIG. 7, a wireless device is
connected to a PCell in the licensed band and one or more SCells in
the unlicensed band. In this application we denote a secondary cell
in unlicensed spectrum as licensed-assisted access secondary cell
(LAA SCell).
[0032] The use of LTE carrier aggregation (CA), introduced since
Rel-10, offers a way to increase the peak data rate, system
capacity and user experience by aggregating radio resources from
multiple carriers that may reside in the same band or different
band.
[0033] In Rel-13, LAA (Licensed-Assisted Access) has attracted much
interest in extending the LTE carrier aggregation feature towards
capturing the spectrum opportunities of unlicensed spectrum in the
5GHz band. WLAN operating in the 5GHz band already supports 80MHz
in the field and 160MHz is to follow in Wave 2 deployment of IEEE
802.11ac. Enabling the utilization of multi-carrier operation on
unlicensed carrier using LAA is deemed necessary as further CA
enhancements. The extension of the CA framework beyond 5 carriers
has been started in LTE Rel-13. The objective is to support up to
32 carriers in both UL and DL.
[0034] The existing contention window adaptation protocols are
based on the reception of a single automated repeat request (ARQ)
feedback value (ACK/NACK) that is received after the transmission
of a burst of data. In the case of LTE, first a hybrid ARQ (HARQ)
protocol is followed instead of a simple ARQ protocol. Thus,
multiple retransmissions based on HARQ feedback may be needed
before a single ARQ feedback value at the higher layer is
generated. Furthermore, in LTE the HARQ feedback is only available
after a delay of 4 ms which corresponds to multiple subframes.
Existing solutions assume the feedback is available after a very
short time interval after the transmission ends. Thus, these
solutions do not effectively deal with a system like LTE where the
feedback delay is much larger. How to adapt contention window sizes
in a multi-carrier setting has also not been defined yet for
LAA.
[0035] On each LAA carrier, multiple wireless devices may be
scheduled for reception or transmission by an eNB in a single
subframe. In addition, a single LAA transmission may consist of
multiple subframes. Finally, a transmission to or from a single
wireless device may have multiple HARQ feedback values if the
transmission is a multi-codeword transmission. Thus, there are
multiple ways in which multiple feedback values may be received
corresponding to a single transmission burst following a successful
channel contention. A central problem is how multiple HARQ feedback
values corresponding to different component carriers are used in
determining the contention window size(s) for the next channel
contention.
SUMMARY
[0036] Some embodiments advantageously provide a method and system
for adaptation of contention windows in a multicarrier wireless
communication system implementing a listen-before-talk protocol.
According to one aspect, a method includes determining at least one
component carrier, CC, of multiple CCs to serve as a backoff
channel The method further includes performing a listen-before-talk
procedure on the at least one CC serving as a backoff channel The
listen-before-talk procedure includes sensing for each backoff
channel whether a clear channel exists during a backoff period
drawn from a contention window, CW. The LBT procedure also includes
deferring transmitting on a CC for which the sensing does not
indicate that a clear channel exists. The LBT procedure also
includes transmitting on a CC for which the sensing indicates a
clear channel exists. The method also includes determining a size
of the CW based on at least one transmission feedback value.
[0037] According to this aspect, in some embodiments, the
listen-before-talk procedure further includes performing a clear
channel assessment, CCA, on CCs that do not serve as a backoff
channel and transmitting on CCs for which a CCA indicates a clear
channel. In some embodiments, only one CC serves as a backoff
channel In some embodiments, the transmission feedback value is a
Hybrid Automatic Repeat Request, HARQ, transmission feedback value
and the CW is increased only if a ratio of negative
acknowledgments, NACKs, to acknowledgements, ACKs, on each
component carrier, CC, exceeds a threshold. In some embodiments,
the transmission feedback value is a Hybrid Automatic Repeat
Request, HARQ, transmission feedback value and the CW is increased
only if a ratio of negative acknowledgments, NACKs, to
acknowledgements, ACKs, on all backoff channels exceeds a
threshold. In some embodiments, an increase of the size of the CW
is obtained by multiplication of a CW by a factor greater than one.
In some embodiments, the backoff period is drawn from a joint
contention window, JCW, that is determined from CWs of the multiple
component carriers. In some embodiments, the JCW is a maximum of
the CWs of the multiple component carriers. In some embodiments,
the JCW is an average of the CWs of the multiple component
carriers. In some embodiments, the CW is increased if at least one
transmission on a CC results in a negative-acknowledgment, NACK,
signal.
[0038] According to another aspect, a network node for adaptation
of contention windows in a multicarrier wireless communication
system implementing a listen-before-talk protocol is provided. The
network node includes processing circuitry that includes a
processor and a memory. The memory is in communication with the
processor and includes executable instructions that, when executed
by the processor, configure the processor to perform functions for
adaptation of contention windows. The processor is further
configured to determine at least one component carrier, CC, of
multiple CCs to serve as a backoff channel and to perform a
listen-before-talk procedure on the at least one CC serving as a
backoff channel The listen-before-talk procedure includes sensing
for each backoff channel whether a clear channel exists during a
backoff period drawn from a contention window, CW, deferring
transmitting on a CC for which the sensing does not indicate that a
clear channel exists, and determining a size of CW based on at
least one transmission feedback value. The network node further
includes a transmitter configured to transmit on CCs for which the
sensing indicates a clear channel exists.
[0039] According to this aspect, in some embodiments, the
listen-before-talk procedure further includes performing a clear
channel assessment, CCA, on CCs that do not serve as a backoff
channel, and transmitting on CCs for which a CCA indicates a clear
channel. In some embodiments, only one CC serves as a backoff
channel In some embodiments, the transmission feedback value is a
Hybrid Automatic Repeat Request, HARQ, transmission feedback value
and CW is increased only if a ratio of negative acknowledgments,
NACKs, to acknowledgements, ACKs, on each backoff channel exceeds a
threshold. In some embodiments, the transmission feedback value is
a Hybrid Automatic Repeat Request, HARQ, transmission feedback
value and the CW is increased only if a ratio of negative
acknowledgments, NACKs, to acknowledgements, ACKs, on all backoff
channel exceeds a threshold. In some embodiments, an increase of
the size of the CW is obtained by multiplication of a CW by a
factor greater than one. In some embodiments, the backoff period is
drawn from a joint contention window, JCW, that is determined from
CWs of the multiple component carriers. In some embodiments, the
JCW is a maximum of the CWs of the multiple component carriers. In
some embodiments, the JCW is an average of the CWs of the multiple
component carriers. In some embodiments, the CW is increased if at
least one transmission on a CC results in a
negative-acknowledgment, NACK, signal.
[0040] According to yet another aspect, a network node for
adaptation of contention windows in a multicarrier wireless
communication system implementing a listen-before-talk protocol is
provided. The network node includes a listen-before-talk module,
configured to perform a listen-before-talk, LBT, procedure for each
of at least one component carrier, CC, serving as a backoff
channel, to determine whether a clear channel exists on the backoff
channel during a backoff period drawn from a contention window, CW.
The network node also includes a contention window size
determination module configured to determine a size of the CW based
on at least one transmission feedback value. The network also
includes a transmitter module configured to transmit on CCs for
which the sensing indicates a clear channel exists, and defer
transmitting on CCs for which the sensing does not indicate that a
clear channel exists.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] A more complete understanding of the present embodiments,
and the attendant advantages and features thereof, will be more
readily understood by reference to the following detailed
description when considered in conjunction with the accompanying
drawings wherein:
[0042] FIG. 1 is a time frequency grid;
[0043] FIG. 2 is a time domain representation of frames and
subframes;
[0044] FIG. 3 is a time frequency grid showing control signaling
and reference symbols;
[0045] FIG. 4 illustrates aggregated carriers;
[0046] FIG. 5 illustrates an LBT process;
[0047] FIG. 6 illustrates a process for load based clear channel
assessment;
[0048] FIG. 7 illustrates transmission on a licensed carrier and an
unlicensed carrier for a wireless device;
[0049] FIG. 8 is a block diagram of an embodiment of a network node
constructed in accordance with principles set forth herein;
[0050] FIG. 9 is a block diagram of an alternative embodiment of a
network node constructed in accordance with principles set forth
herein;
[0051] FIG. 10 illustrates a first embodiment of a random back off
procedure;
[0052] FIG. 11 illustrates a second embodiment of a random back off
procedure;
[0053] FIG. 12 illustrates a third embodiment of a random backoff
procedure;
[0054] FIG. 13 illustrates a fourth embodiment of a random backoff
procedure;
[0055] FIG. 14 illustrates a fifth embodiment of a random backoff
procedure;
[0056] FIG. 15 illustrates a sixth embodiment of a random backoff
procedure; and
[0057] FIG. 16 is a flowchart of an exemplary process for
adaptation of contention windows.
DETAILED DESCRIPTION
[0058] Before describing in detail exemplary embodiments, it is
noted that the embodiments reside primarily in combinations of
apparatus components and processing steps related to adaptation of
contention windows in multi-carrier listen-before-talk (LBT)
protocols. Accordingly, components have been represented where
appropriate by conventional symbols in the drawings, showing only
those specific details that are pertinent to understanding the
embodiments so as not to obscure the disclosure with details that
will be readily apparent to those of ordinary skill in the art
having the benefit of the description herein.
[0059] As used herein, relational terms, such as "first" and
"second," "top" and "bottom," and the like, may be used solely to
distinguish one entity or element from another entity or element
without necessarily requiring or implying any physical or logical
relationship or order between such entities or elements.
[0060] The term "network node" used herein may refer to a radio
network node, e.g., a core network node, mobile switching center
(MSC), mobile management entity (MME), operation and maintenance
(O&M), operation support system (OSS), self-organizing network
(SON), positioning node (e.g., enhanced serving mobile location
center (E-SMLC)), mobile data terminal (MDT) node, etc. The term
network node, as used herein, may also include a wireless device,
such as a user equipment (UE), in a cellular or mobile
communication system. Examples of a wireless device are target
device, device to device (D2D) wireless device, machine type
wireless device or wireless device capable of machine to machine
(M2M) communication, PDA, iPAD, Tablet, mobile terminals, smart
phone, laptop embedded equipped (LEE), laptop mounted equipment
(LME), USB dongles etc.
[0061] The term "radio network node" used herein can be any kind of
network node comprised in a radio network which may further
comprise any of base station (BS), radio base station, base
transceiver station (BTS), base station controller (BSC), radio
network controller (RNC), evolved Node B (eNB or eNodeB), Node B,
multi-standard radio (MSR) radio node such as MSR BS, relay node,
donor node controlling relay, radio access point (AP), transmission
points, transmission nodes, Remote Radio Unit (RRU) Remote Radio
Head (RRH), nodes in distributed antenna system (DAS), etc.
[0062] Various embodiments address ways to adapt contention windows
used in LBT for channel access during multi-carrier operation. The
embodiments describe various methods by which contention window
sizes can be varied based on one or more HARQ feedback values from
one or more component carriers. Separate embodiments address the
cases where there are individual random backoff processes on each
carrier, and when there is a single random backoff process located
on one of the available carriers.
[0063] A description of the proposed contention window variation
techniques for LBT protocols follows. This is generally applicable
for both downlink (DL) and uplink (UL) transmissions, for both
frequency division duplex (FDD) and time division duplex (TDD)
systems. Thus, embodiments described herein may be implemented in a
wireless device or base station or other network node. These are
referred to generally as network nodes. In the following, the
contention window from which a random backoff counter can be drawn
for a new LBT attempt is represented by CW so that the random
backoff counter drawn falls within [0, C]. The default contention
window size is denoted by CWmin. In the event that the CW is
increased, in one example the current CW is doubled to obtain the
new upper limit on the CW. A maximum value of CWmax may be imposed
on the CW, such that the size of CW.ltoreq.CWmax always.
[0064] Some embodiments address LBT for data transmissions that are
carried for example on the PDSCH or PUSCH of a particular CC, and
in general can be per beam of a particular CC. The receiver of a
data transmission may provide HARQ feedback to the transmitter to
indicate whether the data has been received successfully (ACK) or
not (NACK) according to LTE specifications. The NACK here
corresponds to the scheduled data. The contention window size is
modified by the transmitter based on the HARQ feedback. In some
embodiments, the modifications are based on all the previously
unused HARQ feedback received that are available at the time the
LBT operation is performed to access the channel
[0065] As a non-limiting example, a HARQ feedback value for a
transmission in subframe n is assumed to be available for use in
adjusting a LBT procedure that occurs at a time no earlier than
n+5, i.e., at least five subframes later. This is because of the 4
ms HARQ feedback delay currently used in LTE, as well as any
additional processing delay. The HARQ feedback delay may be reduced
in future releases of LTE and embodiments described herein may be
adapted accordingly. A transmission burst refers to a transmission
by a network node performed after a successful channel contention.
The transmission burst may have one or more subframes with each
subframe having transmissions to one or more users and the
transmissions to one or more users containing one or more codewords
that can each receive individual HARQ feedback. Each transmission
burst must be preceded by a successful LBT procedure where the
network node determines the channel to be free to transmit.
[0066] The following embodiments can be easily extended for the
multi-beam transmission per CC. Also, in the multicarrier
transmissions, the following embodiments can be considered for a
group or set of the carriers where the number of groups or sets of
carriers and their corresponding members can be changed or be
reconfigurable. A set or a group can have one or more component
carriers or one or more beams. Different groups can apply one
method or combination of methods described in the following
embodiments. The methods can be the same or different for different
groups and can be changed by time.
[0067] A first embodiment considers the multi-carrier scenario with
multiple backoff channels at a network node. In the case of
multiple backoff channels, the network node that wants to access
the channel performs LBT on each carrier with either the same
random backoff counter, or different random backoff counters drawn
from within the same CW. A random backoff counter determines the
backoff period during which the network node does not transmit. The
network node then transmits on the corresponding carriers where LBT
succeeds and defers transmission on carriers which did not finish
the backoff procedure. CCs that have completed their backoff may
defer their transmission until other, additional CCs also complete
their backoff.
[0068] FIG. 8 is a block diagram of a network node 20 configured to
perform adaptation of contention window size in a multicarrier
wireless communication system implementing a listen-before-talk
protocol. The network node 20 has processing circuitry 21. In some
embodiments, the processing circuitry may include a memory 24 and
processor 22, the memory 24 containing instructions which, when
executed by the processor 22, configure processor 22 to perform the
one or more functions described herein. In addition to a
traditional processor and memory, processing circuitry 21 may
comprise integrated circuitry for processing and/or control, e.g.,
one or more processors and/or processor cores and/or FPGAs (Field
Programmable Gate Array) and/or ASICs (Application Specific
Integrated Circuitry).
[0069] Processing circuitry 21 may comprise and/or be connected to
and/or be configured for accessing (e.g., writing to and/or reading
from) memory 24, which may comprise any kind of volatile and/or
non-volatile memory, e.g., cache and/or buffer memory and/or RAM
(Random Access Memory) and/or ROM (Read-Only Memory) and/or optical
memory and/or EPROM (Erasable Programmable Read-Only Memory). Such
memory 24 may be configured to store code executable by control
circuitry and/or other data, e.g., data pertaining to
communication, e.g., configuration and/or address data of nodes,
etc. Processing circuitry 21 may be configured to control any of
the methods described herein and/or to cause such methods to be
performed, e.g., by processor 22. Corresponding instructions may be
stored in the memory 24, which may be readable and/or readably
connected to the processing circuitry 21. In other words,
processing circuitry 21 may include a controller, which may
comprise a microprocessor and/or microcontroller and/or FPGA
(Field-Programmable Gate Array) device and/or ASIC (Application
Specific Integrated Circuit) device. It may be considered that
processing circuitry 12 includes or may be connected or connectable
to memory, which may be configured to be accessible for reading
and/or writing by the controller and/or processing circuitry
21.
[0070] The network node 20 may be a wireless device such as a
mobile phone, or may be a base station, e.g., eNodeB. In some
embodiments, the processing circuitry 21 may be implemented by
application specific integrated circuitry or a programmable gate
array. In some embodiments, the memory may include a contention
window size determiner 26 and a listen-before-talk unit 28. The
network node 20 may also include a transmitter 30. The contention
window size determiner 26 is configured to determine a size of a
contention window (CW) based on at least one Hybrid Automatic
Repeat (HARQ) feedback value. The LBT unit 28 is configured to
perform sensing via backoff channel sensing unit 32 for each of
multiple component carriers (CCs) to determine whether a clear
channel exists on a carrier during the CW. The sensing may be a LBT
procedure. The transmitter 30 is configured to transmit on CCs for
which the LBT procedure indicates a clear channel exists; and defer
transmitting via transmission deferral unit 34 on CCs for which the
LBT procedure does not indicate that a clear channel exists.
[0071] FIG. 9 is a block diagram of an alternative embodiment of a
network node 40 configured to perform adaptation of contention
window size in a multicarrier wireless communication system
implementing a listen-before-talk protocol. The network node 40
includes a contention window size determination module 42 and a LBT
module 44. These modules may be implemented as software modules
containing instructions that, when executed by a processor,
configure the processor to perform functions describe herein. The
network node 40 may also include a transmitter module 46 configured
to transmit on CCs for which the LBT procedure indicates that a
clear channel exists.
[0072] Operation of the network node 20, 40 is discussed below by
way of examples. Of note, although embodiments are discussed with
respect to network node 20, it is understood that the embodiments
are equally implementable using network node 40. An illustrative
example of DL multi-carrier LBT with two CCs on the unlicensed band
and each CC performing its own random backoff procedure is shown in
FIG. 10. All CCs share a common CW. In the figure, each CC performs
a successful LBT procedure via the listen before talk unit 28 with
the same random backoff counter at subframe 0, followed by a
transmission burst of four subframes via of transmitter 30. The
listen before talk procedure implemented by the LBT unit 28
includes backoff channel sensing via backoff channel sensing unit
32 and transmission deferral via transmission deferral unit 34. On
the first transmission time interval (TTI) of the burst, wireless
device (WD) WD 1 and WD 2 are scheduled by SCell 1, while on SCell
2, WD 3 and WD 4 are scheduled for single codeword and
multiple-codeword reception, respectively. The HARQ feedback values
corresponding to these transmissions are available to the
transmitting eNB by the start of the LBT procedure prior to the
next intended transmission burst at subframe 6. HARQ feedback
values for subframe 1 of the same transmission burst are not
available, and therefore are not used to determine the CW for the
next LBT phase at subframe 6. The LBT procedure for the next
transmission burst therefore uses all the previously unused HARQ
feedback values that are available at that point.
[0073] Note that HARQ feedback values for different codewords
transmitted to a single user in a subframe may be combined to form
a single effective HARQ feedback value, for example using a logical
AND or a logical OR operation. In addition, in some embodiments,
one may use only the HARQ feedbacks corresponding to the scheduled
users for the intended LBT. HARQ feedback values may be combined in
additional ways across users and subframes.
[0074] In the following list of exemplary implementations of this
embodiment, a binary exponential backoff scheme is assumed where
the CW for the next LBT procedure is doubled when it is determined
that it should be increased. In some embodiments, the rate of CW
increase may not be binary exponential, i.e., instead of being
doubled it may be scaled by some factor greater than 1. The CW size
is determined by the contention window size determiner 26.
[0075] In a first example, the size of the CW is doubled if any
wireless device on any CC has a NACK.
[0076] In a second example, the size of the CW is doubled only if
all CCs have at least one wireless device with a NACK.
[0077] In a third example, the size of the CW is doubled only if
the ratio of NACKs to ACKS on each CC exceeds a threshold.
[0078] In a fourth example, the size of the CW is doubled only if
the ratio of NACKs to ACKS on any CC exceeds a threshold.
[0079] In a fifth example, the size of the CW is doubled only if
the ratio of NACKs to ACKs across all CCs combined exceeds a
threshold.
[0080] In a sixth example, the size of the CW is doubled only if
the ratio of CCs that have one or more NACK exceeds a
threshold.
[0081] In a seventh example, the size of the CW is doubled only if
the ratio of NACKED CCs exceeds a threshold. A NACKED CC is a CC
that has a ratio of NACKS to ACKS that is higher than a
threshold
[0082] In an eighth example, the size of the CW is doubled only if
a certain number `K` of CCs has a ratio of NACKs to ACKS that
exceeds threshold. The number of CC can be parameterized and
adaptive depending on the number of carriers in congestions. For
instance, if there are many carriers in congestion, K can be
reduced. In the opposite case, it can be increased to have more
conservative CW doubling.
[0083] In a ninth example, the size of the CW is doubled only if a
certain number `K` of CCs has one or more NACKs. The number of CC
can be parameterized and adaptive depending on the number of
carriers in congestions. For instance, if there are many carriers
in congestion, K can be reduced. In the opposite case, it can be
increased to have more conservative CW doubling.
[0084] The next set of exemplary implementations describes how to
decrease the CW based on HARQ feedback values.
[0085] In a first example, the size of the CW is reset to CWmin if
all HARQ feedback values across all CCs are ACKs.
[0086] In a second example, the size of the CW is reset to CWmin if
each backoff channel has a number of ACKs greater than some
threshold.
[0087] In a third example, the size of the CW is reset to CWmin if
the number of ACKs across all CCs combined exceeds a threshold.
[0088] In a fourth example, the size of the CW is reset to CWmin
only if a certain number `K` of CCs have a number of ACKs greater
than some threshold. The number of CC can be parameterized and
adaptive depending on the number of carriers in congestions. For
instance, if there are many carriers in congestions, K can be
increased. In the opposite case, it can be decreased to be more
aggressive in resetting CW.
[0089] In a fifth example, the size of the CW is reset to CWmin
after the CWmax is used for a certain number of time intervals,
consecutively.
[0090] A second embodiment addresses a multi-carrier scenario with
multiple backoff channels at a network node 20. In the case of
multiple backoff channels, the network node 20 that wants to access
the channel performs LBT on each carrier with either the same
random backoff counter, or different random backoff counters drawn
from within the same CW. The network node 20 then transmits on the
corresponding carriers where LBT succeeds and defers transmission,
via transmission deferral unit 34, on carriers which did not finish
the random backoff procedure. CCs that have completed their backoff
may defer their transmission until other, additional CCs also
complete their backoff.
[0091] In this embodiment, a contention window, CWi, is maintained
for a channel i, separately from other channels, based on the CW
tracking protocol for a single channel as is known in the art. The
said same or different random backoff counters to be used in the
LBT for the carriers is or are drawn from a joint contention window
(JCW) that is determined by the CWs of these carriers.
[0092] In one exemplary implementation, the JCW is the max of the
CWi. For instance,
JCW=max(CW1, CW2, . . . , CW5)
[0093] in the case of five LBT channels.
[0094] In another exemplary implementation, the JCW is the linear
average of the CWi. For instance,
JCW = CW 1 + CW 2 + + CW 5 5 ##EQU00001##
[0095] in the case of five LBT channels.
[0096] In yet another exemplary implementation, the JCW is the
harmonic average of the CWi. For instance,
JCW = ( 1 CW 1 + 1 CW 2 + + 1 CW 5 ) - 1 ##EQU00002##
in the case of five LBT channels.
[0097] In a further exemplary implementation, the JCW is the max of
the CWi. For instance,
JCW = CW 1 .times. CW 2 .times. .times. CW 5 5 ##EQU00003##
[0098] in the case of five LBT channels.
[0099] In another exemplary implementation, the JCW is a weighted
average of the CWi. For instance,
JCW = w 1 .times. CW 1 + w 2 .times. CW 2 + + w 5 .times. CW 5 w 1
+ w 2 + + w 5 ##EQU00004##
[0100] in the case of five LBT channels. The weight w.sub.i for a
channel i reflects the aspects of the operation conditions of the
channel i. [0101] In one embodiment, the weight wi for a channel i
reflects the bandwidth of the channel. [0102] In another
embodiment, the weight w.sub.i for a channel i reflects the average
noise and interference level of the channel. [0103] In yet another
embodiment, the weight w.sub.i for a channel i reflects the
transmission power of the channel.
[0104] An illustrative example of DL multi-carrier LBT with two CCs
on the unlicensed band and each CC performing its own random
backoff procedure is shown in the embodiment of FIG. 11. In the
embodiment of FIG. 11, each CC maintains a separate CW, CW1 and
CW2. In contrast, in the embodiment of FIG. 10, each CC performs a
successful LBT procedure with the same random backoff counter at
subframe 0, followed by a transmission burst of four subframes. In
FIG. 11, on the first TTI of the burst, WD 1 and WD 2 are scheduled
by SCell 1, while on SCell 2, WD 3 and WD 4 are scheduled for
single codeword and multiple-codeword reception, respectively. The
HARQ feedback values corresponding to these transmissions are
available to the transmitting eNB by the start of the LBT procedure
prior to the next intended transmission burst at subframe 6. In
addition, in some embodiments, one may use only the HARQ feedbacks
corresponding to the scheduled users for the intended LBT. New CW 1
and CW 2 are computed based on the HARQ feedbacks for the two
channels. The JCW is determined based on CW 1 and CW 2. A random
backoff counter may be drawn based on JCW and supplied to both
channels for LBT operations.
[0105] A third embodiment addresses the multi-carrier scenario with
a single backoff channel In a first method of this third
embodiment, the network node 20 performs LBT with full-fledged
random backoff on only one of the multiple carriers. A short time
before the expected completion of the random backoff process, the
network node 20 does a quick CCA check on the other carriers, and
transmits on the backoff channel plus a subset of the other
carriers that are determined to be free based on the quick CCA
check. The carriers on which the quick CCA check is performed are
denoted as ancillary carriers. As a non-limiting example, the
duration of the quick CCA check on the ancillary carriers may be
equal to or larger than the PIFS duration of Wi-Fi (generally 25
.mu.s).
[0106] An illustrative example of a single backoff channel and one
ancillary CC is shown in the embodiment of FIG. 12, where the sole
backoff channel is placed on SCell 1, and SCell 2 is operated as an
ancillary carrier. SCell 1 conducts LBT tests with full random
backoff at subframes 0 and 6 in this example, while the ancillary
CC conducts a quick CCA check that is initiated by the backoff
channel. The CW used in the LBT process prior to the transmission
burst from subframe 6 takes into account the HARQ feedback values
from subframe 0 of both CCs in the previous transmission burst. The
scheme described above can be applied to an arbitrary number of
ancillary CCs that need not be contiguous in frequency. Note that
HARQ feedback values may be combined to obtain an effective
feedback value as described above with respect to the first
embodiment.
[0107] In a second method of the third embodiment the network node
runs parallel full random backoff on multiple carriers with a
common CW and when one is completed first, the channel is monitored
for a short time before completion of that carrier. If the channel
is found to be idle, a corresponding random backoff is truncated.
Otherwise, the random backoff is continued. The network node 20 in
fact performs LBT with full-fledged random backoff on some of the
carriers where those carriers are all the carriers or a subset of
them. During the random backoff procedures on those carriers, the
carrier which completes the random backoff first, is considered
effectively as the carrier with full-fledge random backoff. As in
the first method, the outcome of a quick CCA check on all other
carriers may be utilized. For this purpose, the network node 20
evaluates whether the channel is idle on all other carriers that
are doing full random backoff or does a quick CCA check on the
other carriers without full random backoff. If the network node 20
finds that the other carriers are idle, it truncates their
corresponding random backoff and uses the last CCA(s) as effective
CCA. The network node 20 transmits on the effective backoff channel
plus the other carriers that are found to be free for a short
period of time. Those carriers except the effective random backoff
carrier are denoted as ancillary carriers. As a non-limiting
example, the duration of the short time during which the channel is
idle, and/or quick CCA check on the ancillary carriers is
performed, may be equal to or larger than the PIFS duration of
Wi-Fi (generally 25 ps). Thus, in some embodiments, for each
channel that is not a backoff channel, the eNB senses the channel
for at least a sensing interval T=25 .mu.s immediately before
transmitting on the backoff channel. Therefore, CCA on CCs that do
not serve as a backoff channel should be performed immediately
before transmitting on the backoff channel
[0108] An illustrative example of an effective single backoff
channel and one ancillary CC is shown in FIG. 13, where both SCell
1 and sCe 112 start with random backoff, where, however, the random
backoff of the SCell 1 completes first. The SCell 2 finds the
channel is idle for a short backoff period of time as part of its
random backoff and truncate its random back off and can be
considered as an effective CCA. Hence, the sole effective backoff
channel is placed on SCell 1, and SCell 2 is operated as an
effective ancillary carrier. LBT tests are performed for
transmissions at subframes 0 and 6. The CW used in the LBT process
prior to the transmission burst from subframe 6 takes into account
the HARQ feedback values from subframe 0 of both CCs in the
previous transmission burst. The scheme described above can be
applied to an arbitrary number of ancillary CCs that need not be
contiguous in frequency. Note that HARQ feedback values may be
combined to obtain an effective feedback value as described above
with respect to the first embodiment.
[0109] A third method is similar to the second embodiment described
above with the difference that every SCell tracks its own CW but
either follows a common rule to adapt the contention window or its
own rule to adapt the contention window. FIGS. 12 and 13 illustrate
these two variants of the third method, respectively. In the
embodiment of FIG. 12, only one backoff channel is present (Scell
1), and a quick CCA is performed on SCell 2. The CW adaptation is
performed for Scell 1 in FIG. 12. In the embodiment of FIG. 13,
there are two backoff channels, on Scells 1 and 2. A common CW is
applied to both these cells. FIG. 14 shows adaptation of the
contention window following a common rule, and FIG. 15 shows
adaptation of the contention window using a rule for each cell.
[0110] In the following list of exemplary implementations of this
embodiment, a binary exponential backoff scheme is assumed where
the size of the CW for the next LBT procedure is doubled when it is
determined that it should be increased. It is to be understood that
the rate of CW increase may not be binary exponential, i.e.,
instead of being doubled it may be scaled by some factor greater
than 1.
[0111] In the first example implementation, the size of the CW is
doubled only if one or more wireless devices report a NACK on the
single backoff channel
[0112] In the second example, the size of the CW is doubled only if
the ratio of NACKs to ACKS on the single backoff channel exceeds a
threshold.
[0113] In the third example implementation, the size of the CW is
doubled if any CC (backoff channel or ancillary channel) has one or
more NACKs.
[0114] In the fourth example implementation, the size of the CW is
doubled if any CC (backoff channel or ancillary channel) has a
ratio of NACKs to ACKS that exceeds threshold. The threshold may be
different for ancillary carriers compared to the single backoff
channel
[0115] In the fifth example implementation, the size of the CW is
doubled only if all CCs (backoff channel and ancillary channels)
have one or more NACKs.
[0116] In the sixth example, the size of the CW is doubled only if
a certain number `K` of CCs (backoff channel and ancillary
channels) has one or more NACKs. The number of CC can be
parameterized and adaptive depending on the number of carriers in
congestions. For instance, if there are many carriers in
congestions, K can be reduced. In the other opposite case, it can
be increased to have more conservative CW doubling.
[0117] In the seventh example, the size of the CW is doubled only
if a certain number `K` of CCs (backoff channel and ancillary
channels) has a ratio of NACKs to ACKS that exceeds threshold. The
number of CC can be parameterized and adaptive depending on the
number of carriers in congestions. For instance, if there are many
carriers in congestions, K can be reduced. In the other In the
opposite case, it can be increased to have more conservative CW
doubling.
[0118] The next set of exemplary implementations describes how to
decrease the CW based on HARQ feedback values when a single backoff
channel is in use. It is to be understood that the rate of CW
decrease may not be being reset to CWmin. Instead, it may be
decrease by some factor greater or less than 1.
[0119] In the first example, the size of the CW is reset to CWmin
if the single backoff channel receives one or more ACKs.
[0120] In the second example, the size of the CW is reset to CWmin
if the backoff channel and all ancillary carriers have a number of
ACKs greater than some threshold. The threshold may be different
for ancillary carriers compared to the single backoff channel.
[0121] In the third example, the size of the CW is reset to CWmin
only if a certain number `K` of CCs (backoff channel and ancillary
channels) have a number of ACKs greater than some threshold. The
number of CC can be parameterized and adaptive depending on the
number of carriers in congestions. For instance, if there are many
carriers in congestions, K can be increased. In the opposite case,
it can be decreased to be more aggressive in resetting CW. In the
fourth example, the CW is reset to CWmin after the CWmax is used
for a certain number of time consecutively.
[0122] In the first example, the size of the CW is reset to CWmin
if the single backoff channel receives one or more ACKs.
[0123] In the second example, the size of the CW is reset to CWmin
if the backoff channel and all ancillary carriers have a number of
ACKs greater than some threshold. The threshold may be different
for ancillary carriers compared to the single backoff channel.
[0124] In the third example, the size of the CW is reset to CWmin
only if a certain number `K` of CCs (backoff channel and ancillary
channels) have a number of ACKs greater than some threshold. The
number of CC can be parameterized and adaptive depending on the
number of carriers in congestions. For instance, if there are many
carriers in congestions, K can be increased. In the opposite case,
it can be decreased to be more aggressive in resetting CW. In the
fourth example, the size of the CW is reset to CWmin after the
CWmax is used for a certain number of times, consecutively.
[0125] FIG. 16 is a flowchart of an exemplary process for
adaptation of contention windows in a multicarrier wireless
communication system implementing a LBT protocol via the listen
before talk unit 28. In some embodiments, a step of the process
includes performing sensing via backoff channel sensing unit 32 for
each of multiple component carriers, CCs, to determine whether a
clear channel exists on a CC during the CW (block S100). If a clear
channel exists, (block S102), the process may include transmitting
on CCs for which the LBT procedure of the listen before talk unit
28 indicates a clear channel exists (block S104). If a clear
channel does not exist, (block S102), the process may also include
deferring transmitting via transmission deferral unit 34 on CCs for
which the LBT procedure does not indicate that a clear channel
exists (block S106). The process also includes determining a size
of a contention window(CW) via the contention window size
determiner 26 based on at least one Hybrid Automatic Repeat, HARQ,
feedback value (block S108).
[0126] Further embodiments include:
[0127] Embodiment 1. A method of adaptation of contention windows
in a multicarrier wireless communication system implementing a
listen-before-talk protocol, the method comprising:
[0128] sensing for each of multiple component carriers, CCs, to
determine whether a clear channel exists on a CC during a backoff
period drawn from a contention window, CW (S100);
[0129] transmitting on CCs for which the sensing indicates a clear
channel exists (S104); and
[0130] deferring transmitting on CCs for which the sensing does not
indicate that a clear channel exists (S106); and
[0131] determining a size of a CW based on at least one Hybrid
Automatic Repeat, HARQ, feedback value (S108).
[0132] Embodiment 2. The method of Embodiment 1, wherein the CW is
increased if at least one transmission on a CC results in a
negative-acknowledgment, NACK, signal.
[0133] Embodiment 3. The method of Embodiment 1, wherein the CW is
increased only if a ratio of negative acknowledgments, NACKs, to
acknowledgements, ACKs, on at least one of a plurality of component
carriers exceeds a threshold.
[0134] Embodiment 4. The method of Embodiment 1, wherein a rate of
increase of the size of the CW is binary.
[0135] Embodiment 5. The method of Embodiment 1, wherein a rate of
increase of the size of the CW is scaled by a factor greater than
1.
[0136] Embodiment 6. The method of Embodiment 1, wherein the
backoff period is drawn from a joint contention window, JCW, that
is determined from CWs of multiple component carriers.
[0137] Embodiment 7. The method of Embodiment 6, wherein the JCW is
a maximum of the CWs of the multiple component carriers.
[0138] Embodiment 8. The method of Embodiment 6, wherein the JCW is
an average of the CWs of the multiple component carriers.
[0139] Embodiment 9. An apparatus for adaptation of contention
windows in a multicarrier wireless communication system
implementing a listen-before-talk protocol, the apparatus
comprising:
[0140] processing circuitry (21) including:
[0141] a processor (22);
[0142] a memory (24) in communication with the processor, the
memory (24) including executable instructions that, when executed
by the processor (22), configure the processor (22) to:
[0143] sense for each of multiple component carriers, CCs, to
determine whether a clear channel exists on a carrier during a
backoff period drawn from a contention window, CW (S100); and
[0144] determine a size of the CW based on at least one Hybrid
Automatic Repeat, HARQ, feedback value; and
[0145] a transmitter (30) configured to:
[0146] transmit on CCs for which the sensing indicates a clear
channel exists (S104); and
[0147] defer transmitting on CCs for which the sensing does not
indicate that a clear channel exists (S106).
[0148] Embodiment 10. The apparatus of Embodiment 9, wherein the CW
is increased if at least one transmission on a CC results in a
negative-acknowledgment, NACK, signal.
[0149] Embodiment 11. The apparatus of Embodiment 9, wherein the CW
is increased only if a ratio of negative acknowledgments, NACKs, to
acknowledgements, ACKs, on at least one of a plurality of component
carriers exceeds a threshold.
[0150] Embodiment 12. The apparatus of Embodiment 9, wherein a rate
of increase of the size of the CW is binary.
[0151] Embodiment 13. The apparatus of Embodiment 9, wherein a rate
of increase of the size of the CW is scaled by a factor greater
than 1.
[0152] Embodiment 14. The apparatus of Embodiment 9, wherein the
backoff period is drawn from a joint contention window, JCW, that
is determined from CWs of multiple component carriers.
[0153] Embodiment 15. The apparatus of Embodiment 14, wherein the
JCW is a maximum of the CWs of the multiple component carriers.
[0154] Embodiment 16. The apparatus of Embodiment 14, wherein the
JCW is an average of the CWs of the multiple component
carriers.
[0155] Embodiment 17. An apparatus for adaptation of contention
windows in a multicarrier wireless communication system
implementing a listen-before-talk protocol, the apparatus
comprising:
[0156] a listen-before-talk module (44), configured to perform a
listen-before-talk, LBT, procedure for each of multiple component
carriers, CCs, to determine whether a clear channel exists on a
component carrier during a backoff period drawn from a contention
window, CW (S100); and
[0157] a contention window size determination module (42)
configured to determine a size of the CW based on at least one
Hybrid Automatic Repeat, HARQ, feedback value (S108);
[0158] a transmitter module (46) configured to:
[0159] transmit on CCs for which the sensing indicates a clear
channel exists (S104); and
[0160] defer transmitting on CCs for which the sensing does not
indicate that a clear channel exists (S106).
[0161] Embodiment 18. The apparatus of Embodiment 17, wherein the
CW is increased if at least one transmission on a CC results in a
negative-acknowledgment, NACK, signal.
[0162] Embodiment 19. The apparatus of Embodiment 17, wherein the
CW is increased only if a ratio of negative acknowledgments, NACKs,
to acknowledgements, ACKs, on at least one of a plurality of
component carriers exceeds a threshold.
[0163] Embodiment 20. The apparatus of Embodiment 17, wherein a
rate of increase of the size of the CW is binary.
[0164] Embodiment 21. The apparatus of Embodiment 17, wherein a
rate of increase of the size of the CW is scaled by a factor
greater than 1.
[0165] Embodiment 22. The apparatus of Embodiment 17, wherein the
backoff period is drawn from a joint contention window, JCW, that
is determined from CWs of multiple component carriers.
[0166] Embodiment 23. The apparatus of Embodiment 22, wherein the
JCW is a maximum of the CWs of the multiple component carriers.
[0167] Embodiment 24. The apparatus of Embodiment 22, wherein the
JCW is an average of the CWs of the multiple component
carriers.
[0168] As will be appreciated by one of skill in the art, the
concepts described herein may be embodied as a method, data
processing system, and/or computer program product. Accordingly,
the concepts described herein may take the form of an entirely
hardware embodiment, an entirely software embodiment or an
embodiment combining software and hardware aspects all generally
referred to herein as a "circuit" or "module." Furthermore, the
disclosure may take the form of a computer program product on a
tangible computer usable storage medium having computer program
code embodied in the medium that can be executed by a computer. Any
suitable tangible computer readable medium may be utilized
including hard disks, CD-ROMs, electronic storage devices, optical
storage devices, or magnetic storage devices.
[0169] Some embodiments are described herein with reference to
flowchart illustrations and/or block diagrams of methods, systems
and computer program products. It will be understood that each
block of the flowchart illustrations and/or block diagrams, and
combinations of blocks in the flowchart illustrations and/or block
diagrams, can be implemented by computer program instructions.
These computer program instructions may be provided to a processor
of a general purpose computer, special purpose computer, or other
programmable data processing apparatus to produce a machine, such
that the instructions, which execute via the processor of the
computer or other programmable data processing apparatus, create
means for implementing the functions/acts specified in the
flowchart and/or block diagram block or blocks.
[0170] These computer program instructions may also be stored in a
computer readable memory or storage medium that can direct a
computer or other programmable data processing apparatus to
function in a particular manner, such that the instructions stored
in the computer readable memory produce an article of manufacture
including instruction means which implement the function/act
specified in the flowchart and/or block diagram block or
blocks.
[0171] The computer program instructions may also be loaded onto a
computer or other programmable data processing apparatus to cause a
series of operational steps to be performed on the computer or
other programmable apparatus to produce a computer implemented
process such that the instructions which execute on the computer or
other programmable apparatus provide steps for implementing the
functions/acts specified in the flowchart and/or block diagram
block or blocks.
[0172] It is to be understood that the functions/acts noted in the
blocks may occur out of the order noted in the operational
illustrations. For example, two blocks shown in succession may in
fact be executed substantially concurrently or the blocks may
sometimes be executed in the reverse order, depending upon the
functionality/acts involved. Although some of the diagrams include
arrows on communication paths to show a primary direction of
communication, it is to be understood that communication may occur
in the opposite direction to the depicted arrows.
[0173] Computer program code for carrying out operations of the
concepts described herein may be written in an object oriented
programming language such as Java.RTM. or C++. However, the
computer program code for carrying out operations of the disclosure
may also be written in conventional procedural programming
languages, such as the "C" programming language. The program code
may execute entirely on the user's computer, partly on the user's
computer, as a stand-alone software package, partly on the user's
computer and partly on a remote computer or entirely on the remote
computer. In the latter scenario, the remote computer may be
connected to the user's computer through a local area network (LAN)
or a wide area network (WAN), or the connection may be made to an
external computer (for example, through the Internet using an
Internet Service Provider).
[0174] Many different embodiments have been disclosed herein, in
connection with the above description and the drawings. It will be
understood that it would be unduly repetitious and obfuscating to
literally describe and illustrate every combination and
subcombination of these embodiments. Accordingly, all embodiments
can be combined in any way and/or combination, and the present
specification, including the drawings, shall be construed to
constitute a complete written description of all combinations and
subcombinations of the embodiments described herein, and of the
manner and process of making and using them, and shall support
claims to any such combination or subcombination.
[0175] It will be appreciated by persons skilled in the art that
the embodiments described herein are not limited to what has been
particularly shown and described herein above. In addition, unless
mention was made above to the contrary, it should be noted that all
of the accompanying drawings are not to scale. A variety of
modifications and variations are possible in light of the above
teachings without departing from the scope of the following
claims.
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